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Designed by Hanle and Prasser, the 17m Kestrel high performance Open Class single-seater was designed to meet the demand for a successor to the Libelle variants with a longer wing span and roomier cockpit; it was known originally as the 17m Libelle and has a new fuselage and wing profile and a T-tail. The fuselage is a fiberglass monocoque (not sandwich) for greater resilience and pilot protection. The cockpit has room enough for a 198 cm / 6 ft pilot and features a control stick that slides fore and aft (rather than rotating) to reduce likelihood of pilot induced oscillations and which has a press-to- trim-push-button trimmer. It has a nose and a center of gravity release.

The cantilever two piece shoulder wings are of glassfibre and balsa and/or foam sandwich construction, with unidirectional glassfibre spar caps and glassfibre and balsa shear webs. To cure a small airflow separation problem at the wing root fuselage junction at low speeds large wing root fillets were added to production aircraft; these were actually developed by Vickers-Slingsby, who built the Kestrel 17 under licence. High lift camber-changing flaps are featured which operate in conjunction with the ailerons between 8° and +12°, and can be lowered to 35° for a landing; both ailerons and flaps are partially mass-balanced. It features airbrakes, drogue chute, water ballast and retractable gear. Up to 99lb of water ballast can be carried. There are flush fitting air brakes on the wing upper surfaces, and also a tail braking parachute which can be streamed for short-field landings.

The Kestrel 17 tail unit is similar in construction to the wings, the fixed T-tail being secured by three attachments. Both the elevator and rudder are mass balanced. The monowheel is retractable, with an internally expanding brake, and there is an interchangable tailwheel or tailskid.

The prototype Kestrel first flew at Karlsruhe-Forchheim on 9 August 1968 and production deliveries began the following year, reaching a total of 129 by January 1978.

The Kestrel 17 has several records to its credit. Goran Ax won the second Smirnoff Transcontinental Sailplane Race in a Kestrel in 1973. Sue Martin of Australia gained both the womens world 100 km speed record (113.2 kph / 70.36 mph ) and 300 km triangle speed record (114.4 kph/ 71.11 mph). The 100km closed circuit speed record of 102.74mph was set by K. B.Briegleb of the USA on 18 July 1974 (since broken by an AS-W 17), and the ladies' 300km closed circuit speed record of 71.1mph set up by Susan Martin of Australia on 11 February 1972.

Glasflugel 401 Kestral 17

The Italian firm of Glasfaser Italiana SrL has also built 25 Kestrel 17s, as well as 130 complete fuselage assemblies for the Kestrel.

After Slingsby Aircraft Company Ltd had gone into liquidation in July 1969 the firm was reorganised as part of the Vickers Group, at first as Slingsby Sailplanes, later trading as Vickers-Slingsby and then as Slingsby Engineering Ltd. It was decided in September 1969 to produce a modern glassfibre design, and a licence to build the Kestrel was negotiated with Glasflugel. Construction of the first Slingsby-built T 59 Kestrel 17 began in March 1970, and it first flew on 15 August that year at Rufforth, Yorkshire.

After putting the Glasflugel Kestrel 17 into production, Vickers-Slingsby (later Slingsby Engineering Ltd) began to pursue its own line of development of this Open Class single-seater, which resulted in the 19m span T59B, T59C and T59D. The first 19m span Kestrel, the T 59B, being flown by Mr G Burton in the 1970 World Championships in Texas. The 19m T 59C, which had a carbon-fibre main spar, first flew on 7 May 1971. This was followed by the T 59D of the same span, which first flew in July 1971, the extra span being in the form of 0.5m at each wing root and 0.5m at each wing tip. The T 59D also featured a larger fabric-covered rudder and an antibalance tab in the elevator.

The D model was still further developed into the T59H of 22m (72ft 2.25in) span, the extra span consisting of two 1.5m stub wings inserted into the exisiting wing at the roots. The fuselage is similar to the Kestrel 17 up to just aft of the canopy, beyond which an additional section 29 1/2 in long is inserted which considerably reduced the 'waisting' of the earlier version. The fin and rudder area are increased by about 25%, although the tailplane is the same size as the Kestrel 17's; the rudder is lightened to prevent flutter by fabric-covered cut-out sections.

Two prototype T59Hs were built, the first of these flying in 1974, but the new variant was found to suffer from a wing flutter at 140kts (161mph); Vickers-Slingsby had to recover the prototype T59H from the original customer who had bought it, while the flutter problem was investigated by the College of Aeronautics, where it was still being studied early in 1978. Like the T59C, the H has a carbon-fibre main spar and, apart from the longer fuselage and long-span four-piece wing, joined at the flap/aileron junction, it is structurally similar to the Kestrel 17 with the same cantilever T-tail, up to 220lb of water ballast can be carried. There are Schempp-Hirth air brakes in the upper and lower wing surfaces, and there is a retractable unsprung monowheel with a disc brake, plus a fixed tailwheel. The two T59H prototypes are known as the Kestrel 22 Series 1 and Series 2.

A total of 101 Kestrels had been built by Vickers-Slingsby by the end of 1974, plus two 22m span T 59H Kestrel 22s. Most of these have been 17m span versions.